✨ Add network MTU variable into cluster-class

[michal-gubricky]

What this PR does / why we need it:
This PR adds an option to specify network MTU via variable in your cluster resource

Which issue(s) this PR fixes (optional, in fixes #<issue number>(, fixes #<issue_number>, ...) format, will close the issue(s) when PR gets merged):
Fixes #149

Special notes for your reviewer:
Tested with cluster.yaml file, which looks like:

apiVersion: cluster.x-k8s.io/v1beta1
kind: Cluster
metadata:
  name: cs-cluster
  labels:
    managed-secret: cloud-config
spec:
  clusterNetwork:
    pods:
      cidrBlocks:
        - 192.168.0.0/16
    serviceDomain: cluster.local
    services:
      cidrBlocks:
        - 10.96.0.0/12
  topology:
    variables:
      - name: controller_flavor
        value: "SCS-2V-4-50"
      - name: worker_flavor
        value: "SCS-2V-4-50"
#      - name: external_id
#        value: ebfe5546-f09f-4f42-ab54-094e457d42ec # gx-scs
      - name: network_mtu
        value: 1420
    class: openstack-scs-1-28-v0-sha.wenvkai
    controlPlane:
      replicas: 1
    version: v1.28.11
    workers:
      machineDeployments:
        - class: default-worker
          failureDomain: nova
          name: default-worker
          replicas: 3

After the cluster was spawned successfully, in the gx-scs infrastructure, we are able to see changed MTU for the cluster network

Please confirm that if this PR changes any image versions, then that's the sole change this PR makes.

TODOs:

• squash commits
• include documentation
• add unit tests